Just an update, in case someone wanted to give me some help!
I tried some tricks but to no avail:
- adding "acpi_osi=Linux acpi_backlight=vendor" to the line GRUB_CMDLINE_LINUX_DEFAULT in /etc/default/grub and updating grub after it;
- also tried to change "acpi_backlight=vendor" with the value "kernel" or "legacy"
- also tried to change "acpi_osi=Linux" with the value "!Windows 2012"
I tried all the possible combinations of these and didn't work.
Moreover, I tried playing around with the files in /sys/class/backlight/acpi_video0 , namely echoing a value to the file "brightness", but nothing happens.
I'm officially stuck with no solutions!